Nickel has excellent corrosion resistance in alkaline conditions, and also exhibits excellent corrosion resistance under a high concentration chloride environment. Accordingly, a nickel material is utilized for forming a member (a seamless tube, a welded tube, a plate material or the like) in a variety of chemical plants such as facilities for making caustic soda or vinyl chloride.
In these facilities, nickel materials are utilized after being welded in many parts.
A nickel material includes carbon (C) as an impurity element. However, a solubility limit of C in nickel is low. Accordingly, if a nickel material is used for a long time under a high temperature, C precipitates in grain boundaries. Further, when a nickel material is welded, C may precipitate in grain boundaries due to the heat effect of welding. In these cases, there may be a case where a nickel material is embrittled, thus reducing corrosion resistance.
ASTM B161 “Standard Specification for Nickel Seamless Pipe and Tube” and ASTM B163 “Standard Specification for Seamless Nickel and Nickel Alloy Condenser and Heat-Exchanger Tubes” specify that the C content in a normal nickel material is 0.15% or less. The normal nickel material is designated as UNS No.: N02200 in the above-mentioned ASTM standard, for example. However, for the application of a nickel material used for a long time under a high temperature, a nickel material where the C content is further reduced has been put into use. The nickel material where the C content is further reduced is designated as UNS No.: N02201 in terms of the above-mentioned ASTM standard, for example. The C content in N02201 is 0.02% or less.
However, also in a nickel material having a low C content such as N02201, there may be a case where, in long-term use under a high temperature, C included in the nickel material as an impurity precipitates in grain boundaries (grain boundary precipitation), thus reducing corrosion resistance.
International Application Publication No. WO 2008/047869 (Patent Literature 1) discloses a technique for suppressing, in a nickel material, precipitation of C in grain boundaries under a high temperature.
A nickel material disclosed in Patent Literature 1 includes, in mass %, C: 0.003 to 0.20%, and one, two or more kinds of elements selected from a group consisting of Ti, Nb, V and Ta with a total amount of less than 1.0%, wherein ( 12/48)Ti+( 12/93)Nb+( 12/51)V+( 12/181)Ta−C≥0 is satisfied, and the balance being Ni and impurities. In Patent Literature 1, Ti, Nb, V, Ta and the like are included in a nickel material, and C is stabilized in grains as a carbide. Patent Literature 1 describes that, with such a configuration, precipitation of C in grain boundaries under a high temperature can be suppressed.